Petrology and thermobarometry of mafic granulites and migmatites from the Chafalote Metamorphic Suite: New insights into the Neoproterozoic P– T evolution of the Uruguayan—Sul-Rio-Grandense shield

Abstract

This paper reports a study of the metamorphic evolution of pelitic, semi-pelitic migmatites and mafic granulites of the Chafalote Metamorphic Suite (CMS), Uruguay, which represents the southernmost exposures of high-grade metamorphic rocks in the Dom Feliciano Belt, Uruguain—Sul-Rio-Grandense shield, South America. This belt is one of the Brasiliano orogens that crop out along the Brazilian and Uruguayan Atlantic margin, and the CMS is one of several disconnected segments of supracrustal rock in a dominantly granitic terrain. Petrological evidence from CMS mafic granulites and semi-pelitic migmatites indicates four distinct metamorphic assemblages. The early prograde assemblage (M 1) is preserved only as inclusions in porphyroblasts of the peak-metamorphic (M 2) assemblage. Peak-metamorphism was followed by near-isothermal decompression (M 3), which resulted in symplectites and coronitic textures in the mafic granulites and compositional zoning of Ca in garnet (decreasing rimwards) and plagioclase (increasing rimwards) in the semi-pelitic migmatites. The retrograde metamorphic assemblage (M 4) is represented by hydration reaction textures replacing minerals of the M 2 and M 3 assemblages. Average P– T calculations using the program THERMOCALC and conventional thermobarometric methods yield peak-metamorphic (M 2) P– T conditions of 7–10 kbar and 830–950 °C, near-decompressional (M 3) P– T conditions of 4.8–5.5 kbar and 788–830 °C and M 4 retrograde P– T conditions of 3–6 kbar and 600–750 °C. The calculated P– T path for the CMS rocks is ‘clockwise’ and incorporates a near-isothermal decompression segment followed by minor cooling, consistent with a history of crustal thickening followed by extensional collapse at ca. 650–600 Ma. The metamorphism recorded by rocks of this crustal segment may be correlated with ∼650 Ma metamorphism in the Coastal Terrane of the Kaoko Belt in Namibia, being the first unequivocal match between South America and Africa provided by crystalline rocks south of the Congo Craton.